I second mrmagloo's comment. Anything that can direct more air to the rear cylinder definitely won't hurt. The only issue I've seen with the Harley or aftermarket fans has been reliability. Broken mounting brackets and actual fan motor failures seem to have been fairly common. As for the appearance, that is in the eye of the beholder. Ugly but no damage from overheating is probably better than pretty but broken. Of course, that's not the traditional Harley way of looking at things.
I'm not familiar with the rules in the UK, but if you can retune the bike (new ignition and fuel maps) to run cooler, and refrain from sitting for long periods at idle, you could probably get by fine without the fan. Many people in the US have solved their overheating problem just with a good (but illegal of course) tune.
Jerry